Method and device for the exchange of data between at least two users connected to a bus system

ABSTRACT

A method for the exchange of data in messages between at least two users which are connected by a bus system, the messages containing the data being transmitted by the users via the bus system, and each user containing a predefinable and/or ascertainable timing mark; a first user, in a function as timer, controls the messages as a function of time in such a way that it repeatedly transmits a reference message, which contains time information of the first user, via the bus at a specifiable time interval, and if the timing mark of the second user is reached without a reference message of the first user reaching the second user, the at least second user takes over the function of timer by transmitting its own reference message with its own time information via the bus system.

FIELD OF THE INVENTION

The present invention relates to a method and a device for the exchangeof data between at least two users (nodes) connected to a bus system.

BACKGROUND INFORMATION

As related art in automotive networking, the CAN (controller areanetwork) protocol has been used for several years. In that case, thecommunication is controlled in an event-driven manner. Very great loadscan be produced when the transmission of various messages is to beinitiated at the same time. The non-destructive arbitration mechanism ofCAN guarantees the sequential transmission of all messages according tothe priority of their identifiers or identifications. For hard real-timesystems, an analysis of the transit times and bus loads for the entiresystem is made in advance, to make sure that all message deadlines canbe observed (even under peak load).

There are already communication protocols which are based ontime-controlled processing, such as TTP/C or Interbus-S. The exceptionalfeature in this case is that the bus access is already planned inadvance by allocating transmission instants. Thus, no collisions canoccur during the transit time. However, in the same way, a peak load isavoided on the communications bus. Thus, the bus is frequently notcompletely utilized to capacity.

In the case of the TTP/C protocol, the time information is distributedby allocation of the communication objects to the global time andadjustment of the local time of the individual users by fault-tolerantaveraging of all time information.

Other bus protocols derive the time information from the individual bitsof a message. Consequently, a controlled run-up of the system that is asrapid as possible is not ensured in every case. In the same way, theentire system can collapse if the time-giving user malfunctions.Furthermore, automatic resumption of the function is not always ensuredafter the malfunction.

It is apparent that the related art is unable to deliver optimal resultsin every respect.

SUMMARY OF THE INVENTION

Thus, the present invention advantageously includes a method and adevice for the exchange of data in messages between at least two userswhich are connected by a bus system, the messages containing the databeing transmitted by the users via the bus system, and each usercontaining a predefinable and/or ascertainable timing mark; a firstuser, in a function as timer, controls the messages as a function oftime in such a way that it repeatedly transmits a reference message,which contains time information of the first user, via the bus at aspecifiable time interval, and if the timing mark of the second user isreached without a reference message of the first user reaching thesecond user, the at least second user takes over the function of timerby transmitting its own reference message with its own time informationvia the bus system.

In this context, the CAN bus is used particularly expediently as thebasic bus system or bus protocol. However, the present invention relatesgenerally to any bus system or bus protocol in which an object-orientedmessage transfer or data transfer is used, thus the message and/or thedata contained therein are clearly recognizable by an identifier.Consequently, this is valid for all busses in which not the users, butrather the messages or their data are addressed, in particular the CANbus.

Advantageously, various users, particularly all users, are potentiallyprovided as timers, and these users can transmit their own referencemessages with time information of their own via the bus when the user'sown timing mark has been reached without a reference message having beenreceived.

The messages are advantageously controlled by a first user as a functionof time in such a way that the first user repeatedly transmits areference message via the bus at least one predefinable time interval,and the time interval is subdivided into timing windows of specifiablelength, the messages being transmitted in the timing windows.

Therefore, compared to the related art, the present inventionadvantageously includes a higher protocol layer with respect to theactual bus (CAN) protocol which is retained unchanged within theframework of the time-controlled communication according to the presentinvention. Thus, the time-controlled communication advantageously allowsfull capacity utilization of the bus, and at the same time makes itpossible to hold the latency times for each message to a defined value.

The present invention therefore includes a cyclically proceedingtransfer of bus (CAN) messages. In this manner, a deterministic andcombinable communication system is produced. Such a system issubsequently referred to as TTCAN in the present invention. Furthermore,in the same way, the starting point is a CAN bus, the considerationsbeing valid generally for all bus systems or bus protocols havingobject-oriented message transfer, as mentioned above.

The reference message and the subsequent timing windows up to the nextreference message are expediently combined to form a first cycle ofspecifiable length and/or specifiable structure, the structurecorresponding to the length, number and time position of the timingwindows in the time interval which follow the reference message.

Furthermore, a plurality of first cycles of the same structure areadvantageously combined to form a second cycle, messages in the secondcycle also being repeatedly transmitted in timing windows whose timeinterval is greater than the time length of the first cycle.

One cyclical message transfer is expediently omitted in at least onetiming window of the first or the second cycle. In these initially emptytiming windows, it is then possible to transfer arbitrating messages,thus such which do not have to be transmitted cyclically, but rather areavailable when, for example, certain sequences are concluded.

A priority with respect to the timer function can expediently beassigned to the users potentially used as timer, that useradvantageously initially taking over the function as timer for the firstcycle whose timing mark is reached first without the user havingreceived a reference message, and in the following first cycle of thesecond cycle, that user having the highest priority taking over thefunction as timer. In this manner, the priority of the timers isexpediently configurable (logical sequence).

The aforesaid measures advantageously ensure the timer function and theorderly communications sequence or message exchange over time after theinitialization or in the event of the malfunction of a timer in atime-controlled bus system, particularly a CAN bus system (TTCAN).

A further advantage is the controlled, fastest possible runup of thesystem, composed of users and bus system, obtained in this case. Asystem collapse in response to the malfunction of a timer (node) isexpediently avoided, and automatic resumption of the function is ensuredafter a malfunction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a bus system having a plurality of users.

FIG. 2 shows the sequence in principle of the first cycles or basecycles and the second cycles, the overall cycles over time.

FIG. 3 illustrates in detail the design and message occupancy of thetiming windows.

FIG. 4 then shows an overall cycle having 7 base cycles and diversetransmission groups of messages, as well as arbitrating messages.

FIG. 5 shows, by analogy with FIG. 1, the transfer of the timer functionto a further user which is designed as a potential timer.

FIG. 6 shows, in a state diagram, various possibilities for the transferof the timer function in view of the timer priorities.

DETAILED DESCRIPTION

TTCAN is based on a time-controlled, periodic communication which isclocked by a timer (node, user) with the aid of a time-referencemessage, or reference message RN for short. The period up to the nextreference message RN is denoted as base cycle and is subdivided into ntiming windows (see FIG. 2). Each timing window allows the exclusivetransmission of one periodic message of variable length. These periodicmessages are transmitted in a TTCAN controller by using timing markswhich are coupled to the expiration of a logical relative time. However,TTCAN also allows the consideration of vacant timing windows. Thesetiming windows can be used for so-called spontaneous messages, theaccess within these timing windows to the bus being utilized via thearbitration scheme of CAN (arbitrating messages). The synchronization ofthe timer clock (global time gZ) with the internal local time ofindividual nodes 1Z1 through 1Z4 is taken into account and efficientlyconverted.

FIG. 1 shows a bus system 100 having a plurality of bus users 101through 105. In this context, each user 101 through 105 has a separatetime base 106 through 110 which can be transmitted, on the one hand, byan internal element such as a clock, counter, clock generator, etc., orexternally to the respective user. The respective local time base 1Z1through 1Z4 is, in particular, a counter, for example, a 16-bitincrementing, which may only be influenced by a HW reset. In thisinstance, the local time base is implemented in each node or user 102through 105. One user, the timer, 101, has an exposed setting. Its timebase is designated as global time base 106 having global time gZ, and iseither implemented in timer 101, or is transmitted to it externally.Global time gZ is formed in principle in each node from local time base107 through 110, i.e. local time 1Z (1Z1 through 1Z4) and an offset OS1through OS4. As a rule, this offset OSg for timer 101 is equal to zero(OSg=0). All other nodes form their view of global time gZ from localtime 1Z (1Z1 through 1Z4) and local offset OS1 through OS4 and OSg, whenOsg≠0. The case when OSg is not zero occurs, for example, when globaltime gZ is transmitted from the outside to timer 101 which, in addition,contains its own time base 106. Then, the timer is also calibrated toglobal time gZ, and gZ and time base 106 may not agree. The local offsetis the difference between the local time at the transmission instant(SOF, start of frame) of the reference message, and the global timetransmitted by the timer in this reference message.

Local Time Base and the Global Time

Local Time Base: The local time base is a counter, such as a 16-bitincrementing, which may only be influenced by a HW reset. The local timebase is implemented in each node.

Reference-Mark Buffer Register: In response to each assumed SOF, thebuffer register is loaded with the local time base.

Reference Marker: If the instantaneous message is recognized as areference message, then the value is retrieved from the buffer registerinto the reference marker (as local reference mark). The referencemarker is configured, for example, as a 16-bit register.

Timer Reference Mark: This is the reference mark of the timer receivedby the time takers in the reference message.

Local Offset With Respect To the Global Time: The local offset to theglobal time is the difference between the reference mark in the bufferregister and the global timing mark received in the reference message.It is used for calculating the global time from the local time. Theoffset of the timer itself remains constant. The timer transmits itslocal reference mark plus the local offset in the reference message.

Thus, timer 101 is also that node or user which transmits time-referencemessage 111, i.e., reference message RN for short. Arrow 112 indicatesthat reference message RN 111 is dispatched, especially simultaneously,to remaining users 102 through 105.

Reference message RN is the basis for the time-controlled, periodicoperation of TTCAN. It is clearly denoted by a special identifier, andis received by all nodes, here 102 through 105, as clock generator. Inprinciple, it is sent out cyclically by timer 101.

The reference message can include the following data: The number of theinstantaneous base cycle BZn, the reference mark of the timer in globaltime.

The reference mark is formed by the takeover of the internal counterreading at the instant of the “start of frame” bit (SOF) upon receptionof the reference message of the timer. Thus, the reference mark is aninstantaneous recording of the local time base at the instant ofreceiving the reference message. Relative time RZ1 through RZ4 and RZgspecified in the users is the difference between the local time base andthe last reference mark. All definitions with regard to the timing marksused relate to the relative time of an individual user. For example, itcan be permanently available as a signal (e.g., by combining the tworegister values via gates). The reference mark determines the relativetime of all nodes on the TTCAN bus.

Watchdog Wg and W1 through W4, likewise shown, is a special relativeinstant. Such a relative instant (watchdog)—at which a new referencemessage, and thus a reference mark, as well, is expected at thelatest—is defined in each node. Therefore, the watchdog represents aspecial timing mark. The watchdog is used primarily in the initializingand re-initializing to monitor whether a communication has taken placeat all. In this case, the watchdog should always be greater then theinterval between the reference messages.

In this context, one timing mark is a relative instant which establishesthe relationship between the relative time and an action in the originalbus (CAN) controller. One timing mark is represented as a register, acontroller having the ability to manage a plurality of timing marks. Aplurality of timing marks can be allocated to one message (see, forexample, in FIG. 4: transmission group A occurs both in timing windowZF1 a, and in timing window ZF4 a).

With regard to the application, an application watchdog is actuated inparticular. This watchdog is employed regularly by the application inorder to signal the proper operation to the TTCAN controller. Themessages are sent by the CAN controller only when this watchdog isactuated.

FIG. 2 shows the principle of the time-controlled, periodic message ordata transfer over time. This message transfer is clocked by the timerwith the aid of the reference message. Time interval t0 through t6 isdesignated as base cycle BZ and is subdivided into k timing windows(kεN). Reference messages RN of respective base cycles BZ0 through BZ3are transmitted from t0 to t1, t6 to t7, t12 to t13 and t18 to t19, thusin timing window ZFRN. The structure of timing windows ZF1 through ZF5following a reference message RN, thus their length (in segments S,where Δts=tsb−tsa), their number and their time position, isspecifiable. In this manner, an overall cycle GZ1 which begins at t0 andends at t24, to be run through anew, can be formed from a plurality ofbase cycles of the same structure. For example, the timing windowsinclude 2 to 5 segments having, for instance, 32 bit times each. Thenumber of timing windows is, for example, 2 to 16, only one timingwindow or more than 16 also being possible. The number of base cycles inan overall cycle is 2^(m), for instance, where, in particular m≦4.

By way of example, tzff1 and tzff2 denote two transmission enablingintervals or timing-window enabling intervals which, for instance, last16 or 32 bit times and describe the time frame within which it ispossible to begin with the transmission of the message with respect tothe base cycle.

Each timing window allows the exclusive transmission of a periodicmessage of variable length. By way of example, FIG. 3 shows two messagesof different length and the allocation in the timing window. Forinstance, message 1 (N1) as block 300 contains 130 bits, and message 2(N2) as block 301 contains 47 bits.

As already mentioned, maximum and minimum timing windows can bepredefined as a function of the message length, for instance, between 2and 5 segments per timing window in this example. Thus, a maximum timingwindow ZFmax is predefined as block 302 which includes 5 segments (S1through S5) having 32 bit times each, and a minimum timing window ZFminis predefined as block 303 which includes 2 segments (S1 and S2) having32 bit times each. Messages N1 and N2 are transmitted in them, themessages thus not having to completely fill up the timing windows;rather, the timing-window sizes are specified according to the messagelength. Therefore, ZFmax offers sufficient time or place for the longestpossible message, e.g. 130 bits or bit times, and ZFmin can be adaptedto the shortest possible message, e.g. 47 bits.

In general, the timing window is the time frame available for a specificmessage (see FIG. 3). The timing window of a message is opened with theapplication of the proceed-to-send signal, and the beginning of thiswindow agrees in principle with a defined timing mark. The length of thetiming window is determined from i segments having, for example, 32 bittimes (see block 304 a). In this context, the segmenting at, inparticular, 32 bit times represents a HW-friendly size. The timingwindow may not be shorter than the longest message occurring in thistiming window. The bit time is, in particular, the nominal CAN bit time.

The transmission enabling interval or timing-window enabling intervaldescribes the time frame within which the transmission of the messagemay be started. The transmission enabling interval is a part of thetiming window. Thus, the proceed-to-send is applied in the intervaltiming mark and timing mark plus delta. The value delta is perceptiblysmaller than the length of the timing window (e.g., 16 or 32 bit timesfor ZFF1 or ZFF2). A message whose beginning does not lie within thetransmission enabling interval may not be sent.

FIG. 4 now represents an overall cycle (transmission matrix) GZ2.Overall Cycle (Transmission Matrix): All messages (RN, A through F andarbitrating) of all users are organized as components of a transmissionmatrix (see FIG. 4). The transmission matrix is made up of individualbase cycles BZ0 a through BZ7 a. All base cycles of overall cycle GZ2have the same structure. These base cycles can be composed selectivelyfrom exclusive (A through F) and arbitrating components. The totalnumber of lines (thus base cycles BZ0 a through BZ7 a) here is a number2^(m)=8, where m=3.

One base cycle (line of the transmission matrix) begins with a referencemark in reference message RN and is composed of a plurality (i) ofsuccessive timing windows of defined length (first timing window ZFO,i.e., ZFRN for RN). The arrangement of the messages within the basecycle can be freely established. A timing window is linked for exclusivecomponents to a CAN message object. A timing window can also be leftfree (409, 421, 441, 417, 445) or be used for arbitrating components(403, 427).

Messages which are always sent in the same timing window but indifferent base cycles form a transmission group (column of thetransmission matrix, A through F) (see FIG. 4). Consequently, a periodcan be established, e.g., A in ZF1 a and ZF4 a: 401, 407, 413, 419, 425,431, 437, 443 and 404, 410, 416, 422, 428, 434, 440, 446. One messageobject (of a timing window) can be sent repeatedly within a transmissiongroup. The period of a message within a transmission group is a number2, where 1<=m.

The message object, i.e., the message, corresponds to the message objectof the bus, particularly in CAN, and includes the identifier, as well asthe data itself. In TTCAN, the message object is supplemented by atleast one, preferably by all three, of the following entries in thetransmission matrix: timing window, base mark, rate of repetition.

The timing window is the position (ZFO, ZF1 a through ZF5 a) in the basecycle (BZn, line of the transmission matrix). The beginning of thetiming window is defined by reaching a specific timing mark.

The base mark indicates in which base cycle (BZ0 a through BZ7 a) in theoverall cycle the message is sent first.

The rate of repetition defines after how many base cycles thistransmission is repeated.

To indicate the validity of a message object for the CAN controller,there is a “permanent transmission request” which signifies a permanentenabling of the object (for exclusive components, see below) and an“individual transmission request” which signifies a one-time validity ofthe object (for arbitrating components, see below).

The automatic retransmission from CAN is advantageously deactivated forthe messages in TTCAN.

In the following, the message transfer—periodic messages and spontaneousmessages—in the base cycle or in the overall cycle, particularly withrespect to the application, is now described again. In so doing,exclusive messages, thus periodic messages, and arbitrating, thusspontaneous messages, are again differentiated.

Exclusive Message Objects (Periodic Messages):

Exclusive message objects are sent when the application watchdog is set,the “permanent transmission request” of the application is set at theCAN controller, and the transmission enabling interval of theappertaining timing window is open. In this case, the timing mark forthe message object agrees with the relative time. The permanenttransmission request remains set until it is reset by the applicationitself.

Arbitrating Message Objects (Spontaneous Messages):

Arbitrating message objects are sent when the application watchdog isset, the “individual transmission request” of the application is set atthe CAN controller, and the transmission enabling interval of the nexttiming window specified for this is open. The timing mark for thistiming window is then equal to the relative time. The transmissionrequest is reset after successful transmission by the CAN controller.The simultaneous access of various spontaneous messages is regulated viathe bit arbitration of CAN. If a spontaneous message in this timingwindow loses versus another spontaneous message, then it can first fightagain for bus access in the next timing window specified for this.

If the entire transmission matrix, i.e. the overall cycle is runthrough, then a cyclical, time-controlled message transmission isyielded. Time-controlled means that each action uses, as a point ofdeparture, the attainment of a specific instant (see timing marks andrelative time). If the overall cycle is run through completely, i.e.,all base cycles were processed once, then a start is made again with thefirst base cycle of the transmission matrix. No time gaps arise in thetransition. An overview of such a time-controlled communications systemwith timer has been presented in the description and the figures of theDrawing.

In a TTCAN system, all timing windows in which messages are transmittedrelate to a reference instant which is communicated to all users by areference sender at the beginning of a base cycle.

FIG. 5 shows a system for that purpose by analogy with FIG. 1. The timeris composed, for example, of a local (e.g., 16-bit wide) counter (of thelocal or global time base), an (e.g., 16-bit wide) offset register, a(e.g., 16-bit wide) reference register and a watchdog timing mark.

In one system, there is a plurality of “potential” timers which, in apredefined priority, are able to send a reference message and thusbecome timers.

During the run-up of the system after switching on, each node providedfor that purpose can assume the timer function.

To that end, the local counter is started and the counter reading isloaded into the reference register. The local counter is running, andeither the watchdog mark is reached, or a reference message is receivedfrom another node.

If the watchdog mark is reached before a reference message has beenreceived, then this node takes over the timer function by sending areference message and thus starting a base cycle.

If the node receives a reference message from another node before thewatchdog mark has been reached, then it synchronizes itself to thisreference and checks whether its own timer priority is higher or lower.

Various possibilities are shown in the state diagram in FIG. 6. In thecase of lower priority, this node continues to function as a normal“potential”, but not active timer.

However, if its timer priority is higher than that of the referencemessage received, as of the next reference instant, it attempts tobecome the timer.

In this context, there are then the following cases:

a) If the higher-priority timer sends the reference message before theprevailing timer, then it takes over the function of timer, and theprevious timer withdraws, since it sees the higher-priority referencemessage.

b) If both timers transmit simultaneously, then the arbitratingmechanism of the CAN protocol comes into effect, and the higher-prioritytimer assumes the function of the timer, since the priority is linked tothe identifier of the reference message.

c) If the higher-priority timer comes too late, then, in a systemwithout drift correction, it attempts to send its reference messageuntil the message gets through, or the timer receives an even higherpriority reference message. In this context, a shift of the global timeis put up with. Drift correction means the adjustment of the accuracy ofthe local clocks distributed in a TTCAN system to synchronizetransmission and reception instants more precisely. When working with asystem having a drift correction, attempt is again made at the nextpossible reference instant to assume the timer function. A suitabledrift-correction algorithm ensures that, at the latest at the secondbase cycle, the higher-priority timer does not place its referencemessage on the bus later than the prevailing timer, and thus wins thearbitration and assumes the timer function.

If the prevailing timer fails, then the watchdogs in the “potential”timers elapse with variable rapidity depending on their priority, andthe node whose watchdog elapses first assumes the timer function (seeabove).

In order that the transition from one timer to the other is as steady aspossible in the time, each timer can transmit its view of the globaltime in its reference message.

The terms time-reference message and reference message are synonymous. Areference message having time information, i.e., the time information inthe reference message is then designated as timer message (see FIG. 6).

1. A method for an exchange of data in messages between at least twousers connected by a bus system, each one of the at least two usersincluding at least one of a predefinable timing mark and anascertainable timing mark, comprising the steps of: causing the at leasttwo users to transmit via the bus system messages including the data;causing a first one of the at least two users, in a function as timer,to control the messages as a function of time such that the first one ofthe at least two users repeatedly transmits a first reference messageincluding a first time information regarding a time base of the firstone of the at least two users, via the bus system at a specifiable timeinterval; if the at least one of the predefinable timing mark and theascertainable timing mark of a second one of the at least two users isreached without the first reference message of the first one of the atleast two users reaching the second one of the at least two users,causing the second one of the at least two users to take over thefunction of timer by transmitting a second reference message with asecond time information via the bus system; subdividing the specifiabletime interval into timing windows of a specifiable length; transmittingthe messages including the data in the timing windows; and combining oneof the first reference message and the second reference message andsubsequent ones of the timing windows up to a next reference message toform a first cycle of at least one of the specifiable length and aspecifiable structure, wherein: the specifiable structure corresponds tothe specifiable length, number and time position of the timing windowsin the specifiable time interval following the one of the firstreference message and the second reference message.
 2. The methodaccording to claim 1, further comprising the steps of: combining aplurality of first cycles of a same specifiable structure to form asecond cycle; and repeatedly transmitting messages in the second cyclein timing windows having a time interval greater than a time length ofthe first cycle.
 3. A method for an exchange of data in messages betweenat least two users connected by a bus system, each one of the at leasttwo users including at least one of a predefinable timing mark and anascertainable timing mark, comprising the steps of: causing the at leasttwo users to transmit via the bus system messages including the data;causing a first one of the at least two users, in a function as timer,to control the messages as a function of time such that the first one ofthe at least two users repeatedly transmits a first reference messageincluding a first time information regarding a time base of the firstone of the at least two users, via the bus system at a specifiable timeinterval; if the at least one of the predefinable timing mark and theascertainable timing mark of a second one of the at least two users isreached without the first reference message of the first one of the atleast two users reaching the second one of the at least two users,causing the second one of the at least two users to take over thefunction of timer by transmitting a second reference message with asecond time information via the bus system; and allocating a prioritywith respect to the function as timer to those of the at least two userscapable of being used as a timer; wherein: initially each one of the atleast two users assumes the function as timer for a first cycle with theat least one of the predefinable timing mark and the ascertainabletiming mark that is reached first without the each one of the at leasttwo users having received a corresponding one of the first referencemessage and the second reference message, and in a following one of thefirst cycle of a second cycle, the one of the at least two users havinga highest priority takes over the function as timer.
 4. The methodaccording to claim 2, further comprising the steps of: omitting acyclical message transfer in at least one of the timing windows of oneof the first cycle and the second cycle; and transmitting arbitratingmessages in the at least one of the timing windows of one of the firstcycle and the second cycle.
 5. A device for an exchange of data inmessages between at least two users connected by a bus system, each oneof the at least two users including at least one of a predefinabletiming mark and an ascertainable timing mark, comprising: means forcausing the at least two users to transmit via the bus system messagesincluding the data; means for causing a first one of the at least twousers, in a function as timer, to control the messages as a function oftime such that the first one of the at least two users repeatedlytransmits a first reference message including a first time informationregarding a time base of the first one of the at least two users, viathe bus system at a specifiable time interval; means for causing, if theat least one of the predefinable timing mark and the ascertainabletiming mark of a second one of the at least two users is reached withoutthe first reference message of the first one of the at least two usersreaching the second one of the at least two users, the second one of theat least two users to take over the function of timer by transmitting asecond reference message with a second time information via the bussystem; means for subdividing the specifiable time interval into timingwindows of a specifiable length; means for transmitting the messagesincluding the data in the timing windows; and means for combining one ofthe first reference message and the second reference message andsubsequent ones of the timing windows up to a next reference message toform a first cycle of at least one of the specifiable length and aspecifiable structure, wherein the specifiable structure corresponds tothe specifiable length, number and time position of the timing windowsin the specifiable time interval following the one of the firstreference message and the second reference message.
 6. The deviceaccording to claim 5, further comprising: means for combining aplurality of first cycles of a same specifiable structure to form asecond cycle; and means for repeatedly transmitting messages in thesecond cycle in timing windows having a time interval greater than atime length of the first cycle.
 7. The device according to claim 6,further comprising: means for omitting a cyclical message transfer in atleast one of the timing windows of one of the first cycle and the secondcycle; and means for transmitting arbitrating messages in the at leastone of the timing windows of one of the first cycle and the secondcycle.
 8. A device for an exchange of data in messages between at leasttwo users connected by a bus system, each one of the at least two usersincluding at least one of a predefinable timing mark and anascertainable timing mark, comprising: means for causing the at leasttwo users to transmit via the bus system messages including the data;means for causing a first one of the at least two users, in a functionas timer, to control the messages as a function of time such that thefirst one of the at least two users repeatedly transmits a firstreference message including a first time information regarding a timebase of the first one of the at least two users, via the bus system at aspecifiable time interval; means for causing, if the at least one of thepredefinable timing mark and the ascertainable timing mark of a secondone of the at least two users is reached without the first referencemessage of the first one of the at least two users reaching the secondone of the at least two users, the second one of the at least two usersto take over the function of timer by transmitting a second referencemessage with a second time information via the bus system; and means forallocating a priority with respect to the function as timer to those ofthe at least two users capable of being used as a timer; wherein:initially each one of the at least two users assumes the function astimer for a first cycle with the at least one of the predefinable timingmark and the ascertainable timing mark that is reached first without theeach one of the at least two users having received a corresponding oneof the first reference message and the second reference message, and ina following one of the first cycle of a second cycle, the one of the atleast two users having a highest priority takes over the function astimer.
 9. The method according to claim 1, further comprising the stepsof: providing each one of the at least two users as timers; and causingthe first one of the at least two users and the second one of the atleast two users to transmit via the bus system the first referencemessage with the first time information and the second reference messagewith the second time information when the at least one of thepredefinable timing mark and the ascertainable timing mark of any of theat least two users has been reached without a receipt of a correspondingone of the first reference message and the second reference message. 10.The method according to claim 1, wherein the messages are cyclicallytransmitted.
 11. The method according to claim 1, further comprising thestep of: allocating a priority with respect to the function as timer tothose of the at least two users capable of being used as a timer. 12.The method according to claim 3, further comprising the steps of:providing each one of the at least two users as timers; and causing thefirst one of the at least two users and the second one of the at leasttwo users to transmit via the bus system the first reference messagewith the first time information and the second reference message withthe second time information when the at least one of the predefinabletiming mark and the ascertainable timing mark of any of the at least twousers has been reached without a receipt of a corresponding one of thefirst reference message and the second reference message.
 13. The methodaccording to claim 3, further comprising the steps of: subdividing thespecifiable time interval into timing windows of a specifiable length;and transmitting the messages including the data in the timing windows.14. The method according to claim 13, wherein the messages arecyclically transmitted.